Technology

About how Beam Profile Reflectometry works and its advantages over comparable metrology and characterisation techniques

Beam Profile Reflectometry

Beam Profile Reflectometry (BPR) is a quantitative and non-destructive technique providing unparalleled accuracy in the ‘on’ and ‘off axis’ characterisation of coatings and membranes. BPR was first used in the early 1990s to characterise thin layers of material on semiconductor wafers. Since then n-eos has taken this base technology and through proprietary enhancements developed a range of metrology products that are helping redefine the measurement of thin layers, coatings and structures on curved or irregular-shaped surfaces.

BPR works by analysing the way that a sample’s reflectance varies as a function of angle. By working at a single wavelength, it avoids the uncertainties that are associated with spectral techniques and enables material properties to be established unambiguously. Prior to the invention of BPR, measuring reflectance as a function of angle involved complex and expensive hardware arrangements where both the light source and detector needed to be moved each time a new angle was selected.

BPR overcomes this difficulty by using a high-magnification lens to bring a collimated laser beam to a sharp focus. As the image on the right shows, at the focal point, which is typically less than 1μm across, light falls on the sample with the whole range of different angles-of-incidence through which the lens bends the light in order to achieve focus. After reflection, the lens recollimates the reflected light and there is a one-to-one correspondence between the physical location of a ray of light within the recollimated beam and the angle at which that ray was reflected from the surface.

When the beam profile is viewed after reflection from a coated surface or membrane, a characteristic ‘bulls eye’ pattern is seen resulting from the interference between rays reflected from the top of the film and those which penetrated to the bottom before being reflected.

Fringe amplitude and period are determined by refractive indices of the materials in the film and film thickness respectively. It is therefore possible to decouple the effects of thickness and refractive index and measure the two classes of parameter independently. It is also possible to measure reflectance as a function of angle of incidence for a wide range of angles (typically, for a ~100X lens, a range of 0-60 degrees) simultaneously, with a very short data acquisition time, using an apparatus with no moving parts.

In the case above the surface is flat and level and the ‘bulls eye’ pattern symmetrical. However if the surface is tilted or curved then a ‘bulls eye’ pattern is still obtained but the centre moves around according to the orientation of the sample surface at the point where the beam is focused. The figure on the right shows an example of this. By locating the fringe centre automatically and taking proper account of the effect of sample tilt upon the fringe patterns, it is possible to obtain accurate measurements even from surfaces which are significantly tilted (typically, up to ±20°) and to measure the surface orientation as well as the thickness and index.

This greatly simplifies the taking of measurements from devices with complex or unpredictable shapes, since so long as the laser spot can be focused upon the sample surface there is no need to ensure that the surface is horizontal.

All measurements take place at a single wavelength, determined by the laser source employed. This is a big advantage over white-light techniques, because there is no need to take account of optical dispersion – the variation of refractive index with wavelength. This means that for each material in the filmstack there is only one value of refractive index to find (or at most two in the case of a birefringent film), and yet there are hundreds of independent data points in the raw data. This data richness enables direct, deterministic measurements of refractive index to be made, in contrast with spectral techniques which are unable to do this and must rely instead on models and assumptions.

To illustrate the advantages and disadvantages of a range of destructive and non-destructive metrology techniques, PA Consulting Group developed a comparison matrix as part of their paper on ‘Approaches to the Physico-Chemical and Mechanical Characterisation of Functional Coatings’ published in the journal Medical Device Technology.

Their comparison clearly illustrates the comparative strength of Beam Profile Reflectometry’s as an effective metrology technology, one with a host of advantages that are distinct and easily transferable across different sectors and disciplines.

To view the matrix on a separate window or download as a PDF please click here.

BPR works by analysing the way that a sample’s reflectance varies as a function of angle. By working at a single wavelength, it avoids the uncertainties that are associated with spectral techniques and enables material properties to be established unambiguously. Prior to the invention of BPR, measuring reflectance as a function of angle involved complex and expensive hardware arrangements where both the light source and detector needed to be moved each time a new angle was selected.

BPR overcomes this difficulty by using a high-magnification lens to bring a collimated laser beam to a sharp focus. As the image on the right shows, at the focal point, which is typically less than 1μm across, light falls on the sample with the whole range of different angles-of-incidence through which the lens bends the light in order to achieve focus. After reflection, the lens recollimates the reflected light and there is a one-to-one correspondence between the physical location of a ray of light within the recollimated beam and the angle at which that ray was reflected from the surface.

When the beam profile is viewed after reflection from a coated surface or membrane, a characteristic ‘bulls eye’ pattern is seen resulting from the interference between rays reflected from the top of the film and those which penetrated to the bottom before being reflected.

Fringe amplitude and period are determined by refractive indices of the materials in the film and film thickness respectively. It is therefore possible to decouple the effects of thickness and refractive index and measure the two classes of parameter independently. It is also possible to measure reflectance as a function of angle of incidence for a wide range of angles (typically, for a ~100X lens, a range of 0-60 degrees) simultaneously, with a very short data acquisition time, using an apparatus with no moving parts.

In the case above the surface is flat and level and the ‘bulls eye’ pattern symmetrical. However if the surface is tilted or curved then a ‘bulls eye’ pattern is still obtained but the centre moves around according to the orientation of the sample surface at the point where the beam is focused. The figure on the right shows an example of this. By locating the fringe centre automatically and taking proper account of the effect of sample tilt upon the fringe patterns, it is possible to obtain accurate measurements even from surfaces which are significantly tilted (typically, up to ±20°) and to measure the surface orientation as well as the thickness and index.

This greatly simplifies the taking of measurements from devices with complex or unpredictable shapes, since so long as the laser spot can be focused upon the sample surface there is no need to ensure that the surface is horizontal.

All measurements take place at a single wavelength, determined by the laser source employed. This is a big advantage over white-light techniques, because there is no need to take account of optical dispersion – the variation of refractive index with wavelength. This means that for each material in the filmstack there is only one value of refractive index to find (or at most two in the case of a birefringent film), and yet there are hundreds of independent data points in the raw data. This data richness enables direct, deterministic measurements of refractive index to be made, in contrast with spectral techniques which are unable to do this and must rely instead on models and assumptions.

To illustrate the advantages and disadvantages of a range of destructive and non-destructive metrology techniques, PA Consulting Group developed a comparison matrix as part of their paper on ‘Approaches to the Physico-Chemical and Mechanical Characterisation of Functional Coatings’ published in the journal Medical Device Technology.

Their comparison clearly illustrates the comparative strength of Beam Profile Reflectometry’s as an effective metrology technology, one with a host of advantages that are distinct and easily transferable across different sectors and disciplines.

To view the matrix on a separate window or download as a PDF please click here.

Advantages of BPR

Compared to other thin film and coating metrology and characterisation techniques, BPR has a number of distinct advantages. These include:

There is no need for prior knowledge about the material’s refractive index (n), since this can be measured simultaneously with the thickness.

There is no need for the orientation of the sample surface to be known prior to measurement: good results are obtained even if the sample surface is tilted by ±20° when data are acquired.

There is no need to scan the laser beam through the sample from one surface to the other, relying on the accuracy of a mechanical stage to obtain the measurement.

Provides refractive index directly – no need to assume or to measure separately.

The largest source of thickness uncertainty is removed, providing true Ångström-to-nanometre accuracy.